Thermocouple Selection Guide

Considerations    Thermocouple Probe Junction Types     Probe Sheath Materials    Definitions    Physical Characteristics of Thermocouples     Maximum Thermocouple Probe Error Limits


Considerations

Extend Your Thermocouples up to 2000 feet without signal loss. Extension wire must be the same type as the thermocouple.

System Error becomes important when you select a probe and meter to make a complete temperature measurement system. For example: a meter has an accuracy of ±0.7°F; from the probe-error limits table below, type T probes with metal sheaths, straight cables, and stripped ends have an error limit of ±1.8°F at 400°F. Therefore, the probe-meter system accuracy will be (±0.7) + (±1.8) = ±2.5°F at 400°F.

NIST Traceability is required for many applications. In order to make an item traceable to NIST standards, the item and the standard are exposed to the same conditions, the readings are noted, and the difference between the readings is recorded on a NIST certificate. When taking future readings with the item, the value on the certificate must be added or subtracted from the measured value.

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Thermocouple Probe Junction Types

Sheaths with small diameters have faster response times; sheaths with larger diameters have longer life and are better for measuring higher temperatures.

Exposed junction

Exposed Junction has the fastest response timeŅideal for measuring rapid temperature changes. Clear coating on most models provides a humidity barrier for the thermocouple. Do not use with corrosive fluids or atmospheres. See table at right for recommended atmosphere type for exposed-junction probes.

Ungrounded junction

Ungrounded Junction has a welded junction insulated from the protective sheath and is electrically isolated. Longer response time; use for conductive solutions or where isolation of the measuring circuitry is required.

Grounded junction

Grounded Junction has a junction welded to tip of sheath. Wires are completely sealed from contaminants. Good response time.

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Probe Sheath Materials

Inconel® 600 Sheath is ideal for severely corrosive environments and at elevated temperatures. Resists progressive oxidation. Maximum operating temperatures: continuous—2100°F, intermittent—2500°F.

304 SS Sheath is for general purpose use, corrosion-resistant, and good for food service and biological applications. Maximum operating temperatures: continuous—1650°F, intermittent—2550°F.

316 SS Sheath has higher corrosion resistance than 304 SS. Withstands some strong acids. Maximum operating temperatures: continuous—1650°F, intermittent—2500°F.

SS Sheath with Coating of PTFE PFA with grounded junction is ideal with corrosive liquids and atmospheres. Longer response time. Temperatures to 500° F (260°C).

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Definitions

Thermocouple Probes are composed of two dissimilar metals, joined to produce a voltage when the applied (measured) temperature differs from the reference temperature.

Thermocouple Thermometers measure, amplify, linearize, and display the proportional voltage signal generated by the thermocouple probe.

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Physical Characteristics of Thermocouples

 

Type J Thermocouple

Wire insulation color:
   + = White
   - = Red
Wire material:
   + = Iron
   - = Constantan
Properties:
   + = Strongly magnetic
Atmosphere for exposed junction:
   Reducing

 

Type K Thermocouple

Wire insulation color:
   + = Yellow
   - = Red
Wire material:
   + = Chromel
   - = Alumel
Properties:
   + = Moderately magnetic
Atmosphere for exposed junction:
   Clean oxidizing

 

Type T Thermocouple

Wire insulation color:
   + = Blue
   - = Red
Wire material:
   + = Copper
   - = Constantan
Properties:
   + = Copper color
Atmosphere for exposed junction:
   Mildly oxidizing and reducing or with moisture

 

Type E Thermocouple

Wire insulation color:
   + = Purple
   - = Red
Wire material:
   + = Chromel
   - = Constantan
Properties:
   + = Greater stiffness
Atmosphere for exposed junction:
   Vacuum, inert mildly oxidizing or reducing

 

Type R Thermocouple

Wire insulation color:
   + = Black
   - = Red
Wire material:
   + = 87% Platinum, 13% Rhodium
   - = Platinum
Properties:
   + = Greater stiffness
Atmosphere for exposed junction:
   Resists oxidation and corrosion, but contaminated by hydrogen, carbon, and metal vapors

 

Type S Thermocouple

Wire insulation color:
   + = Black
   - = Red
Wire material:
   + = 90% Platinum, 10% Rhodium
   - = Platinum
Properties:
   + = Greater stiffness
Atmosphere for exposed junction:
   Resists oxidation and corrosion, but contaminated by hydrogen, carbon, and metal vapors

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Maximum Thermocouple Probe Error Limits

Tolerances apply only to new thermocouples from -200°C to the recommended upper temperature limit of the probe. Tolerances change with use and it is up to the user to establish acceptable limits of error for used thermocouples. Calculated from ASTM tolerances.

Type Maximum error limit
Probes with detachable handles
J, K ±4.0°F (±2.2°C), and ±0.4% of reading above 32°F (0°C); ±2.0% of reading below 32°F (0°C)
T ±1.8°F (±1.0°C), and ±0.4% of reading above 32°F (0°C); ±0.8% of reading below 32°F (0°C)
E ±3.6°F (±2.0°C), and ±0.4% of reading above 32°F (0°C); ±0.5% of reading below 32°F (0°C)
Probes with metal sheath, coiled cord, and connector
J, K ±7.9°F (±4.4°C), and ±0.4% of reading above 32°F (0°C); ±2.0% of reading below 32°F (0°C)
T ±3.6°F (±2.0°C), and ±0.4% of reading above 32°F (0°C); ±0.8% of reading below 32°F (0°C)
E ±6.7°F (±3.7°C), and ±0.4% of reading above 32°F (0°C); ±0.5% of reading below 32°F (0°C)
Probes with metal sheath, straight cable, and connector
J, K ±5.9°F (±3.3°C), and ±0.4% of reading above 32°F (0°C); ±2.0% of reading below 32°F (0°C)
T ±2.7°F (±1.5°C), and ±0.4% of reading above 32°F (0°C); ±0.8% of reading below 32°F (0°C)
E ±5.4°F (±3.0°C), and ±0.4% of reading above 32°F (0°C); ±0.5% of reading below 32°F (0°C)
Probes with metal sheath, straight cable, and stripped ends
J, K ±4.0°F (±2.2°C), and ±0.4% of reading above 32°F (0°C); ±2.0% of reading below 32°F (0°C)
T ±1.8°F (±1.0°C), and ±0.4% of reading above 32°F (0°C); ±0.8% of reading below 32°F (0°C)
E ±3.6°F (±2.0°C), and ±0.4% of reading above 32°F (0°C); ±0.5% of reading below 32°F (0°C)
Probes made of one piece of thermocouple wire with a connector
J, K ±4.0°F (±2.2°C), and ±0.4% of reading above 32°F (0°C); ±2.0% of reading below 32°F (0°C)
T ±1.8°F (±1.0°C), and ±0.4% of reading above 32°F (0°C); ±0.8% of reading below 32°F (0°C)
E ±3.6°F (±2.0°C), and ±0.4% of reading above 32°F (0°C); ±0.5% of reading below 32°F (0°C)
Thermocouple wire only, no connector
J, K ±2.0°F (±1.1°C), and ±0.4% of reading above 32°F (0°C); ±2.0% of reading below 32°F (0°C)
T ±0.9°F (±0.5°C), and ±0.4% of reading above 32°F (0°C); ±0.8% of reading below 32°F (0°C)
E ±1.8°F (±1.0°C), and ±0.4% of reading above 32°F (0°C); ±0.5% of reading below 32°F (0°C)

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